There are many applications for fumed metal oxides, particularly for fumed silica, of extremely fine particle size. Such applications include fillers for polymers such as rubber, paper coatings (i.e., recording media), the manufacture of optical fibers and quartz glassware, thermal insulation, and chemical-mechanical polishing compositions intended for use in semiconductor manufacturing.
Fumed silica is generally produced by the vapor phase hydrolysis of chlorosilanes, such as silicon tetrachloride, in a hydrogen oxygen flame. Such processes are generally referred to as pyrogenic processes. The overall reaction is:SiCl4+2H2+O2→SiO2+4HCl
Organosilanes also have been used in pyrogenic processes for the production of fumed silica. In the vapor phase hydrolysis of organosilanes, the carbon-bearing fragments undergo oxidation to form carbon dioxide as a by-product along with hydrochloric acid.
In this process, submicron-sized molten spheres of silica are formed. These particles collide and fuse to form three-dimensional, branched, chain-like aggregates, of approximately 0.1-0.5 μm in length. Cooling takes place very quickly, limiting the particle growth and ensuring that the fumed silica is amorphous. These aggregates in turn form agglomerates of 0.5-44 μm (about 325 US mesh). Fumed silicas generally have very high purity, with total impurities in many cases below 100 ppm (parts per million). This high purity makes fumed silica dispersions particularly advantageous for many applications.
Numerous methods have been developed in the art to produce fumed silica via pyrogenic processes. U.S. Pat. No. 2,990,249 describes a process for the pyrogenic production of fumed silica. In accordance with this process, a gaseous feedstock comprising a fuel, such as methane or hydrogen, oxygen, and a volatile silicon compound, such as silicon tetrachloride, wherein the oxygen is present in a stoichiometric or hyperstoichiometric proportion, is fed into a burner supporting a short flame having a ratio of height to diameter of about 2:1 or below. Water formed by the combustion of the fuel in oxygen reacts with the silicon tetrachloride to produce silicon dioxide particles, which coalesce and aggregate to form fumed silica. The effluent from the burner is cooled, and the fumed silica is then collected.
U.S. Pat. No. 4,108,964 describes a process for the pyrogenic production of fumed silica using organosilanes as the silicon-containing component. In accordance with this process, an organosilane, such as methyltrichlorosilane, is volatilized at a temperature above the boiling point of the organosilane. The vaporized organosilane is mixed with a gaseous fuel, such as hydrogen or methane, and an oxygen-containing gas containing from 15-100% oxygen, to form a feedstock. The feedstock is fed to a flame supported by a burner at various flow rates to produce fumed silica. The volume ratios of the individual gas components are reported not to be of critical importance. The molar ratio of the organosilane to the water-forming gases generally is said to be in the range of from 1:0 to 1:12.
Despite the foregoing processes for producing fumed silica, there remains a need for other processes for producing fumed silica, especially processes for producing fumed silica more economically and/or efficiently, wherein the fumed silica exhibits desirable properties such as particle uniformity, dispersibility, and the like.
The invention provides such a process. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.